1. Field of the Invention
The present invention relates to a medium holding apparatus, an image recording apparatus and an image forming apparatus, and more particularly, to the structure of a fixed holding member which fixes and holds, and conveys a recording medium in an image recording apparatus such as an inkjet recording apparatus.
2. Description of the Related Art
As a general image recording apparatus, it is suitable to use an inkjet recording apparatus which forms a desired image on a recording medium by ejecting colored inks from a plurality of nozzles provided in an inkjet head. The recording media used in the inkjet recording apparatus is not just paper media, but also includes media of a plurality of types, such as resin sheet, metal sheet, and the like, and furthermore media of various sizes and thicknesses are used.
Desirably, the conveyance member which fixes and conveys the recording medium has a drum shape or belt shape, or the like. Furthermore, for the method of fixing the recording medium, it is suitable to use an air suction method which fixes and holds the recording medium by applying a suction pressure (negative pressure) to the recording medium from inside the conveyance member, via suction holes which are provided in the surface.
In the air suction method described above, if the suction pressure is insufficient, then there is a possibility of positional displacement of the recording medium, and if the suction pressure is excessive, then there is the possibility of deformation of the recording medium, or the occurrence of image abnormalities caused by the ink droplets which have been deposited on the recording medium being sucked into the recording medium due to the suction pressure, or the like. Furthermore, if a plurality of suction holes are provided in accordance with the maximum size so as to achieve compatibility with a plurality of media sizes, and this plurality of suction holes is suctioned by a common pump, then if there are open suction holes in cases where a recording medium of small size is used, air might leak via the open suction holes giving rise to defective fixing of the recording medium due to insufficient suction pressure. Consequently, various ways have been devised in order to avoid problems of these kinds.
Japanese Patent Application Publication No. 10-175338 discloses an inkjet printer which prevents the dispersion of the suction air flow and avoids ink density variations or bleeding by winding a porous sheet member (a thin nylon woven cloth, or the like) about the surface of a drum in which suction holes are provided.
Japanese Patent Application Publication No. 10-193718 discloses a medium holding apparatus for an inkjet printer in which the interior of a drum is divided into a plurality of negative pressure generating chambers, and the positions of the respective negative pressure generating cambers and the suction holes on the drum surface are formed so as to correspond to the length of the print medium.
In a method which fixes a recording medium to a conveyance member by air suction, there are problems such as those described in (1) to (4) below.    (1) If all of the suction holes perform suction all of the time, then the amount of suctioned air becomes very large. Furthermore, depending on the recording medium, it may be impossible to suction the medium even if only several suction holes are left open.    (2) If a fixing method is adopted in which the recording medium is pressed from the head side, then the fixing member presents an obstacle and it is difficult to shorten the throw distance (the distance between the ejection surface of the head), in other words, to bring the head and recording medium closer together, and good image quality cannot be obtained.    (3) Depressions occur in the portions of the recording medium corresponding to the suction holes (the throw distance changes due to depressions of the recording medium in the portions corresponding to the suction holes), and therefore non-uniformities arise.    (4) Since a suction force acts on the ink droplets which have been deposited on the recording medium, then the ink permeation speed varies between the regions of the suction holes and the region outside the suction holes, and therefore density non-uniformity arises in the image.
The inkjet printer described in Japanese Patent Application Publication No. 10-175338 produces loss in the suction pressure due to the porous sheet member, and therefore it is not possible to achieve a high suction pressure. In particular, if using a thick recording medium or a recording medium of high stiffness, it is extremely difficult to fix the recording medium to a drum having a small radius of curvature.
With the medium holding apparatus for an inkjet printer described in Japanese Patent Application Publication No. 10-193718, the compatible recording medium sizes are determined by the size of the negative pressure generating chambers (the surface area in which the suction holes are provided) and it is difficult to change the compatible sizes.